A gene coding for a 3-methylcholanthrene-induced P-450 enzyme has been subcloned into pBR322. Various parts of the gene have been further subcloned to yield 14 overlapping gene regions. These clones are used to study the chromatin structure of the P-450 gene in control and carcinogen treated rats. Nuclease digestions of nuclei isolated from the liver of these rats indicate that the gene is organized in a nucleosomal conformation, that the DNA of the gene is more susceptible to DNAse I digestion than bulk chromatin, and that a DNAse I hypersensitive region is present near the 5' terminal of the gene.